1. Field of the Invention
This invention relates to a color conversion apparatus and method for compressing and outputting entered saturation data in such manner that the data can be reproduced by an output unit that is to output the data.
2. Description of the Related Art
Color reproduction characteristics differ depending upon the device that handles the color data. A color that can be reproduced in one device cannot always be reproduced in another device. Colors can almost never be matched perfectly across different devices even if color matching processing is applied.
Processing is also executed so as to compress (convert the color of) color data reproducible in one device, apply the compressed color data to another device and reproduce the color data in the other device. However, the color reproduced in the other device will be quite different from that reproduced in the first device.
An object of the present invention is to arrange it so that a color which cannot be reproduced in an output unit will be reproduced so as to be as faithful as possible to the original color.
According to the present invention, the foregoing object is attained by providing a color conversion apparatus comprising: a saturation data input unit (saturation data input means) for inputting saturation data; a compression threshold-value decision unit (compression threshold-value decision means) for deciding a compression threshold value regarding saturation based upon the saturation data that has been input from the saturation data input unit and data representing maximum saturation capable of being reproduced by an output unit; and a compression unit (compression means) for compressing the input saturation data that is equal to or greater than the compression threshold value decided by the compression threshold-value decision unit so as to obtain saturation data representing a saturation reproducible by the output unit, and forgoing compression of the input saturation data that is less than the compression threshold value.
The present invention provides also a method suited the apparatus described above. Specifically, the method comprises the steps of: inputting saturation data; deciding a compression threshold value regarding saturation based upon the saturation data that has been input and data representing maximum saturation capable of being reproduced by an output unit; and compressing the input saturation data that is equal to or greater than the decided compression threshold value so as to obtain saturation data representing a saturation reproducible by the output unit, and forgoing compression of the input saturation data that is less than the compression threshold value.
In accordance with the present invention, the saturation data is input and the compression threshold value is decided based upon the input saturation data and the maximum saturation (boundary saturation) capable of being reproduced by an output unit. If the saturation data is equal to or greater than the compression threshold value, the output unit often will not be able to faithfully reproduce the saturation represented by the input saturation data. This saturation data is therefore compressed so as to be capable of reproduction. If the saturation data is less than the compression threshold value, the saturation represented by the input saturation data is reproducible and, hence, no compression processing is applied.
Since data representing a saturation reproducible by the output unit is not compressed, saturation represented by the original saturation data can be reproduced in the output unit. Data representing a saturation that cannot be reproduced by the output unit is compressed and is not subjected to unnecessary compression. As a result, an image having a saturation near that represented by the original data can be reproduced.
It is preferred that the input saturation data equal to or greater than the compression threshold value be compressed non-linearly so as to obtain saturation data representing a saturation than can be reproduced by the output unit.
Since the input saturation data equal to or greater than the compression threshold value is subjected to non-linear compression, compression processing of the saturation data can be realized while maintaining the original gradation characteristic.
The compression threshold value can be decided based upon saturation data which has a high frequency of occurrence in one frame of the input saturation data and which is in the vicinity of the above-mentioned reproducible saturation within the saturations reproducible by the output unit.
Since the compression threshold value is decided based upon saturation data having a high frequency of occurrence, saturation data having a high frequency of occurrence is not compressed. Data having a high frequency of occurrence represents a conspicuous portion in which the proportion of this data is high even in the original image. Since saturation data in an area in which the proportion of the data is high is not compressed, the saturation of a conspicuous portion does not change.
An arrangement may be adopted in which hue data corresponding to input saturation data is entered, the hue data that corresponds to saturation data not reproducible by the output unit is detected based upon the maximum saturation, and compression processing is applied to the entered saturation data that corresponds to the detected hue data.
Thus, only saturation data not reproducible by the output unit is extracted and compressed. Since no compression is applied to reproducible saturation data, the original saturation can be maintained.